1. Field of the Invention
The present invention relates to an analog switch, and more specifically to an analog switch used in sample-and-hold circuits and transfer gates for use in thin film integrated circuits composed of thin film transistors. In particular, the present invention relates to an analog switch which can be used as pixel switched in an active matrix type liquid crystal display, pixel switches in active matrix type vacuum fluorescent display panel, transfer gates of thin film shift registers.
2. Description of Related Art
Since thin film integrated circuits using thin film transistors have a feature that it can be directly formed on a glass plate, the thin film integrated circuits are actively developed as a key component of an active matrix type liquid crystal display, a contact type image sensor for facsimile, a thermal head of a printer, an input/output device of a liquid crystal shutter, etc. In particular, since the thin film transistors formed of polysilicon (polycrystalline silicon) can operate at a high speed, these thin film transistors can be frequently used in a driving circuit. A typical example of the driving circuit includes a shift register for a scanning circuit, a sample-and-hold circuit for holding a signal, and a transfer gate for transferring or switching a signal. In addition, these circuits include an inverter and an analog switch as an important component.
A basic structure of the analog switch includes a thin film field effect transistor having a current path connected at its end to a signal input terminal and at its other end to one end of a capacitive load (for example, a capacitor for storing a signal electric charge. The other end of the capacitive load is connected to a terminal for a reference voltage. A gate of the thin film field effect transistor is connected to receive a control voltage signal. Thus, by controlling the control voltage signal to the gate of the transistor connected between the signal input terminal and the load, an input signal is transferred to the load, or the input terminal is isolated from the load.
However, the conventional analog switch has a serious problem in which a substantial leakage current flows when the polysilicon thin film field effect transistor is turned off, and a voltage of the stored signal varies due to the leakage current. An ideal condition for the analog switch is that an ON current is large and an OFF current is small. The polysilicon thin film field effect transistor has a characteristic in which when the gate voltage is positive, a large current will flow as a matter of course, and when gate voltage is substantially at zero, the current becomes minimum. However, when the gate voltage is negative, a non-negligible off current will flow, and the larger the negative gate voltage becomes, the larger the off current becomes. This phenomenon is inherent to the polysilicon thin film field effect transistor, and a cause for deterioration in the voltage holding characteristics.
In order to overcome the above mentioned problems, various analog switches have been proposed. However, none of the proposed analog switches could have completely satisfied a required performance. In some application fields, it is required to precisely hold the signal for a relatively long period of time. In such a situation, none of the proposed analog switches sufficiently suppress the leakage current, and an error has frequently exceeded a permissible extent.
In addition, in order to realize a faster operation, the capacitance of the load for the analog switch has to be made as small as possible. However, if the capacitance of the load is made small, the voltage holding characteristics will be sacrificed. Furthermore, for a high speed operation, it is effective to widen a channel width so as to enlarge the ON current. However, if the channel width is widened, the leakage current correspondingly increases, and therefore, becomes difficult to hold the signal voltage. For example, the sample-and-hold circuit is required to have a feature that the sampling time is as short as possible but the holding time is as long as possible. However, this request and other various requests occurring with the speed-up of the circuit cannot be satisfied by the conventional countermeasures.